For lubricating two-stroke cross-head diesel internal-combustion engines installed in large ships or vessels, lubricating oils generally called “marine cylinder lubricating oils” are used, In order to operate the internal-combustion engine smoothly, a predetermined amount of the lubricating oil is supplied into the cylinder at a predetermined position on the cylinder liner. Most lubricating oils used nowadays contain various additives to improve various performances and durability of the lubricating oils.
A typical lubricating oil for two-stroke cross-head diesel marine engines comprises a base oil showing a kinematic viscosity of approximately 22 to 300 mm2/s at 40° C. and additives dispersed or dissolved in the base oil such as an ashless dispersant and an overbased metal-containing detergent, e.g., an overbased calcium sulfonate, i.e., an overbased calcium alkylbenzenesulfonate detergent, an overbased calcium phenate, i.e., an overbased sulfurized alkylphenol calcium salt detergent, and an overbased calcium salicylate, i.e., an overbased alkylsalicylic acid calcium salt detergent. Most two-stroke cross-head diesel marine engines mainly burn petroleum fuel containing sulfur in a high content (generally, 2.5 to 4.0 wt. %) such as C fuel oil, and hence exhaust large amounts of acidic sulfur oxides (particularly, sulfuric acid) produced by combustion of the fuel, In order to neutralize the sulfur oxides, the lubricating oil composition contains overbased: metal-containing detergents and ashless dispersants. Further, the overbased metal-containing detergent and the ashless dispersant not only neutralize the sulfur oxides but also evenly disperse the residues of combustion, such as soot or sludge which is produced by deterioration of the fuel and the lubricating oil, so as to prevent residues from accumulating on the inner parts of the engine such as the piston, the piston groove, and the cylinder liner.
Recently, for the purpose of preventing environmental pollution, a requirement has been proposed to use low-sulfur fuels in two-stroke cross-head diesel marine engines. Accordingly, it is necessary to develop lubricating oils advantageously employable in combination with the low-sulfur fuel. In operating the diesel engine with low-sulfur fuel, the lubricating oil must have excellent thermal stability at high temperatures in order to maintain its abrasion-preventing performance, its wear-preventing performance and its scuffing-preventing performance.
In the case where overbased metal-containing detergents such as overbased calcium sulfonates, calcium salicylates and calcium phenates are used in combination in the lubricating oil composition, overbased calcium sulfonates and calcium salicylates are generally used in greater amounts than overbased calcium phenates. For one reason, overbased calcium phenates, are more expensive components than overbased calcium sulfonates and calcium salicylates. Thus, it is more cost effective to use more of the later whenever possible.
U.S. Pat. No. 4,948,522 describes a cylinder lubricating oil composition for marine diesel engines In the examples (set forth in Table 3 of col. 9) of the publication, it is shown that an overbased calcium sulfonate is used in an amount much more than an overbased calcium phenate (weight ratio is approximately 10:1 to 6:1). U.S. Patent Publication 2001/0019999 and U.S. Pat. No. 6,551,965 also describe lubricating oil compositions favorably employable in the two-stroke cross-head diesel marine engines. In contrast with U.S. Pat. No. 4,948,522, the overbased calcium phenate is used in an amount more than the overbased calcium sulfonate (the weight ratio in terms of former:later is 55:45 to 95:5) in U.S. Patent Publication 2001/0019999 and U.S. Pat. No. 6,551,965. The lubricating oil composition further contains an ashless dispersant and a zinc dialkyldithiophosphate and/or a zinc diaryldithiophosphate. It is shown that the lubricating oil composition provides anti-wear performance.
While conventional lubricating oil compositions described in the prior art contain additive compositions comprising overbased metal-containing detergents with satisfactory lubrication performance such as thermal stability, such conventional lubricating oil compositions do not meet the present requirements imposed on two-stroke cross-head diesel marine engines operating on low sulfur fuel that have been recently developed or will be developed in the near future. The new larger bore two-stroke cross-head diesel marine engines run at high outputs and severe loads and the temperature of the cylinder liner are from 220 to 260° C. Further, in consideration of the reducing environmental pollution, it is desired to reduce the amounts of sulfur oxides and nitrogen oxides exhausted by fuel burning in the marine engines. Hence, the trend towards diesel marine engines using fuel having low sulfur content. Accordingly, although conventional lubricating oil compositions for diesel marine engines have a total base number (TBN) of approximately 70 mg·KOH/g, it is now required that the conventional lubricating oil is replaced with a lubricating oil composition having a TBN of approximately 40 mg·KOH/g. There are no available lubricating oil compositions for diesel marine engines having a medium TBN value of 40 mg·KOH/g that can prevent the piston ring and the cylinder liner from wearing and, in particular, inhibit scuffing on the cylinder liner. Thus, there is a need to develop new lubricating oil compositions that can meet the performance demands of such new generation two-stroke cross-head diesel marine engines.